Wastewater and natural waters (e.g., surface water or groundwater) may contain a variety of dissolved inorganic substances from natural and anthropogenic sources. Regulatory limits have been set for a number of these substances in drinking water and for discharges to natural waters, for protection of public health and of environmental quality. The regulatory limits for many of these substances are set at very low levels, e.g., in the range of 2-50 parts-per-billion (“ppb”) or the equivalent units of measure of micrograms-per-liter (“μg/L”).
Conventional water treatment processes, such as co-precipitation with iron or aluminum salts, lime softening, or filtration using adsorbents or ion exchange resins, are ineffective in removing some of these regulated substances to the mandated levels. This problem is of particular concern with respect to certain types of substances including oxyanions, particularly arsenate and arsenite, and some metals, such as mercury, because of their chemistry in water and the particularly low regulatory limits that have been set for them. Typically, the removal of such contaminants can be improved by selecting a treatment matrix (e.g., a co-precipitant or adsorbent) that exhibits a greater capacity to sequester or retain the dissolved substance of concern, or provides more favorable kinetics toward that substance (i.e., the treatment reaction proceeds more quickly). The low capacity or unfavorable kinetics of a treatment matrix can be accommodated to some extent by construction of larger treatment systems to allow the use of larger quantities of the treatment matrix or to provide longer contact times between the treatment matrix and the aqueous stream undergoing treatment. The costs of building and operating such a system increases with the size of the system and often causes such an accommodation to become uneconomical.
A considerable amount of research has been performed to develop metal oxide compounds that can be used economically to attain the low concentrations of dissolved inorganic substances required under current regulations. Some of this research has been directed toward the use of titanium oxides.
U.S. Pat. No. 6,383,395 discloses the use of powdered titanium hydroxide, packed in a column or applied to a filter in the form of a paste, to remove dissolved oxyanions, particularly arsenate, from water.
U.S. Pat. No. 5,618,437 discloses the use of a hydrous titanium oxide dispersed on the surface of a metal oxide supporting matrix to remove dissolved sulfate from a brine.
U.S. Pat. No. 3,332,737 discloses the use of hydrous titanium oxides in packed columns to adsorb several dissolved metals. The hydrous titanium oxides are prepared by treating a solution of a hydrolysable titanium compound with aqueous ammonia or hydrogen peroxide.
Japanese Patent Application Publication 58-45705 discloses the use of hydrous titanium oxides in a slurry to remove oxyanions, such as arsenate, from water at concentrations in the parts-per-billion (ppb) range. The hydrous titanium oxide adsorbent is prepared from a precipitate of a hydrolyzed titanium salt. It is noted in Publication 58-45705 that the kinetics of adsorption are relatively slow, and that a contact time roughly five times as long is required to remove the same amount of arsenate from solution in the absence of certain non-oxygenated acidic anions, such as chloride or sulfide, as when the acidic ions are present.
Japanese Patent Application Publication 57-150481 discloses the removal of arsenate from water by co-precipitation with hydrous titanium oxides. The co-precipitated solids are separated from the aqueous stream after the hydrolysis and co-precipitation reactions have proceeded to completion.
Japanese Patent Application Publication 53-122691 discloses the preparation and use of a composite adsorbent comprising a granular activated carbon and hydrous titanium oxides. The composite adsorbent is prepared by boiling the granular activated carbon in a concentrated solution of a titanium salt in the presence of an oxidative acid, then washing and air-drying the resulting composite adsorbent.